Funny little problem isn't it? Seems common.
My 300E had the exact same problem. What I did was buy some black, dense foam (from a foam sanding block) and I cut it into shapes and inserted it into the space between the plastic cover and the seatbelt mechanism. I've got a black interior so you can't see the foam at all. The foam keeps some pressure on the plastic cover and neither of them rattle anymore - they stay put. I love the silence...
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired
"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon